Transmitting digital data over a telephone network or line requires the use of modems. In the sending modem, a carrier signal is modulated by the data. In the receiving modem, the signal is demodulated to restore the original data.
Because the transmission channel introduces distortion, the modem receiver employs an equalizer for compensating the resulting inter-symbol interference. For every transmission, input amplifier and equalizer must be set to reflect the current transmission characteristics of the channel. Furthermore, carrier synchronization and symbol timing recovery have to be performed.
For setting the parameters in the modem receiver prior to actual data transmission, a so-called training sequence is sent whose elements are known to both, transmitter and receiver. The channel characteristics can then be derived from the received training signal, allowing the receiver to carry out initial gain setting, equalizer training, symbol synchronization, and acquisition of carrier phase and frequency.
The signal level at the receiver input is not known to the modem receiver prior to receiver training. The signal level thus has to be measured by the receiver, and the gain of the receiver input amplifier has to be set so that all signal values are kept within a range acceptable for the units processing these signal values, i.e., preventing signal clipping in the A-to-D converter, and numerical overflow or underflow in receiver filter or equalizer. If more than one amplifier is used, e.g., an analog input amplifier before and a digital amplifier after A-to-D conversion, the gains of both have to be set.
Use of a training sequence was described e.g. in U.S. Pat. No. 4,089,061 (A. T. Milewski) entitled "Method and apparatus for determining the initial values of a complex transversal equalizer". In the system described in this prior art reference, a periodic training signal is used, i.e. one that repeats a basic short sequence periodically.
In some modem receivers, two amplifiers are used: An analog programmable gain amplifier (PGA) for the input signal and a digital amplifier for the digitized samples. The articles by C. Galand et al. "Programmable gain amplifier", IBM Technical Disclosure Bulletin, December 1984, p. 3722 and by C. Couturier et al. "Programmable gain amplifier improvement", IBM Technical Disclosure Bulletin, February 1985, p. 5456, described some features of PGAs. In known modem receivers using a combination of two amplifiers, the gains of the amplifiers were set to a compromise value prior to the reception of a training signal, and then the actual level of a received preliminary test signal was measured. Thereafter, the gains were changed to their appropriate values and reception of an equalizer training signal was started. The samples of the initial test signal already accumulated in filter and equalizer delay lines were discarded after this gain adjustment because they could not be used for equalizer training. This method leads to undesirable delays during modem receiver training, and causes long modem startup times.
Of key interest in modem receiver training is the time required for the training operation. In particular, in multidrop polling systems where the control modem receives many short messages from different tributary modems, the receiver has to adapt to the characteristics of a different telephone channel for each message. The time required for training the modem receiver thus has a strong influence on data throughput. Similarly, when operating in half-duplex transmission, the startup time of the receiving modem contributes significantly to the turn-around delay.